There is continuing interest in reducing the release of carbon dioxide to atmosphere from facilities producing power, heat, fuel and/or chemicals using coal, biomass, petroleum coke, heavy oil, asphalt, natural gas, land fill gas, and other carbonaceous materials. In coal-fired power plants, coal is combusted to generate heat within a boiler to raise steam. The steam is passed into a steam turbine to generate electrical power. In these plants carbon capture is generally implemented as a solvent or sorbent based flue-gas post capture system. Flue gas post capture systems have a large impact on the power plant's overall efficiency due to the energy required in the post capture system for regeneration of the capture media. These systems can also be large and capital intensive due to the very large volume of nitrogen and oxygen contained in the flue gas, and the relatively small concentration of CO2 in the stream (4 to 15% by volume).
The use of oxygen transport membrane (OTM) systems have been contemplated in connection with carbon capture enabled boiler systems to carry out oxy-combustion and produce a flue gas concentrated in CO2. Examples of such systems that can be used to generate electricity are such as those disclosed in U.S. Pat. Nos. 6,394,043; 6,382,958; 6,562,104; and, more particularly U.S. Pat. Nos. 7,856,829 and 8,196,387 and United States patent publication number US 2014/0183866. In such OTM based systems, oxygen is separated from the air with the use of a ceramic membrane that is capable of oxygen ion transport at elevated temperatures. The oxygen ionizes on one surface of the membrane by gaining electrons to form the oxygen ions. Under a driving force of a partial pressure differential, the oxygen ions pass through the membrane and either react with a fuel or recombine to elemental oxygen liberating the electrons used in the ionization of the oxygen.
The advanced power cycle systems disclosed in U.S. Pat. Nos. 7,856,829 and 8,196,387 combust syngas such as that produced in coal gasification systems in oxygen transport membrane reactors configured as oxygen transport membrane boiler. These advanced power cycle systems utilize oxygen transport membrane based partial oxidation (POx) stages operating in a high pressure environment of about 350 psig to directly heat the syngas from the gasifier ahead of the oxygen transport membrane based boiler. One of the recognized problems associated with oxygen transport membranes is that when operating in severe environments that result when combusting fuel at high pressure such as that in oxygen transport membrane based boiler, the reliability of the oxygen transport membranes typically suffer, resulting in more membrane failures and associated system operating downtime and maintenance costs. In addition, oxygen transport membranes that are designed to operate in higher pressure environments typically require very thick support layers thus significantly increasing the cost of the oxygen transport membranes and associated reactors. Therefore, in lieu of operating the oxygen transport membrane boilers at high pressures, these advanced power cycle systems contemplate expanding the heated high pressure gasifier stream to pressure levels as low as near ambient before introduction into the oxygen transport membrane boiler. Regulating or reducing the high pressure gasifier stream involves specialized equipment and adversely impacts the overall economics and efficiency of the oxygen transport membrane based power system. Also, at low pressure the oxygen flux across the membranes is lower, requiring more membrane area, hence higher capital cost.
The electrical power generation method and system described in United States patent publication number US 2014/0183866 first produces a high pressure coal-derived synthesis gas stream by gasifying coal in a gasifier using oxygen from a cryogenic air separation plant; the coal-derived syngas is indirectly heated with radiant heat generated in a partial oxidation oxygen transport membrane reactor stream and expanded to a low pressure that is sufficiently above the atmospheric pressure to overcome the pressure drops in downstream steps. A slip stream of this low pressure syngas is reacted with permeated oxygen in the partial oxygen transport membrane reactor to provide the radiant heat for indirect heating of coal-derived syngas. The reaction products from the partial oxidation oxygen transport membrane reactor and the remaining portion of the low pressure syngas are introduced in an oxygen transport membrane boiler system where these react with permeated oxygen and a source of supplemental oxygen to form carbon dioxide containing flue gas stream while producing steam from a source of boiler feed water. Electric power is generated by expanding the steam in a steam turbine subsystem operatively associated with the oxygen transport membrane based boiler. The carbon dioxide containing flue gas stream is processed to produce a carbon dioxide-rich stream. This oxygen transport membrane based power cycle system facilitates operation of the oxygen transport membrane at low fuel pressures with high fuel utilization or fuel conversion of the high pressure coal-derived synthesis gas and capturing carbon dioxide from oxy-combustion flue gas.
An alternate to boiler based steam cycle power plant is an integrated gasification and combined cycle (IGCC) power generation system in which coal is first converted into a syngas that fuels a gas turbine. In a typical gasifier the carbonaceous feed is reacted with steam and oxygen to produce the syngas. Typically, the oxygen is provided to the gasifier by a cryogenic air separation unit. These IGCC systems can be configured for partial or complete carbon capture.
The IGCC systems configured for full or at least greater than 90% of the carbon capture utilize water gas shift reactors and acid gas removal system to fuel the gas turbine with a hydrogen-rich fuel. For example, in coal-based pre-combustion carbon capture enabled IGCC systems, the syngas produced as a result of the gasification is cooled and further processed in one or more water-gas shift reactors to react carbon monoxide with steam to increase the hydrogen and carbon dioxide content of the syngas. The water-gas shift reactor also hydrolyzes most of the carbonyl sulfide into hydrogen sulfide. The syngas is then further cooled for carbon dioxide and hydrogen sulfide separation within a known solvent scrubbing plant employing physical or chemical absorption for separation of the carbon dioxide and hydrogen sulfides and carbonyl sulfide from the syngas. This allows for the capture and sequestration of the carbon dioxide which is present within the syngas. The resulting hydrogen-rich gas is then fed to a gas turbine that is coupled to an electrical generator to generate electricity. Heat is recovered from the cooling of the raw syngas stream, from cooling the heated discharge from the water-gas shift reactor, and cooling the exhaust from the gas turbine to raise steam. The steam is expanded in a steam turbine to generate additional electrical power.
Systems that employ oxygen blown coal gasification to create a syngas and utilize a water-gas shift and acid-gas separation approach have improved efficiency over the post-capture cases, but are challenged with the capital costs of both oxygen plant, coal gasifier trains, and subsequent clean-up, shift, and separation equipment. Unconverted hydrocarbons, predominantly methane, contained in the syngas from the gasifier is not converted downstream, nor captured by the acid-gas recovery system, and results in a loss of carbon capture efficiency (higher carbon emissions to the atmosphere).
As can be appreciated, the IGCC is environmentally very advantageous in that a clean burning synthesis gas stream is used to power the gas turbine while at the same time, the carbon dioxide produced by the gasification can be captured for use in other industrial processes, for enhanced oil recovery or for sequestration.
There is also considerable interest in utilizing hydrocarbon containing sources with carbon capture. The natural gas, landfill gas, biogas, coke oven gas, process streams available in chemical plants, petroleum refineries, metallurgical plants and the like are some examples of such hydrocarbon containing sources. These sources when used directly in a boiler or gas turbine are amenable to post combustion carbon capture. Another option is to convert these by either reforming or oxidation reactions into a syngas that is further treated in water gas shift reactor and acid gas removal system to produce a hydrogen-rich fuel. This way the carbon dioxide is captured upstream of the boiler or gas turbine.
Steam methane reformers (SMR), oxygen-blown reformers (ATR), partial oxidation reactors (POx) are known to convert methane containing sources into syngas. These, either require fuel-fired furnaces to provide heat for the endothermic reactions, or air separation units (ASU) to supply oxygen. The energy required for these technologies and the additional non-captured carbon resulting from the fuel-fired heaters and furnaces result in energy and carbon capture efficiency penalties.
The present invention proposes the deployment of oxygen transport membranes based oxygen transport membrane syngas reactor at the front end of a carbon-capture enabled system to first convert the feedstock into a synthesis gas that as needed can be further processed in one or more water gas shift reactors and an acid gas removal system to produce a fuel or fuel the integrated gas turbine or oxy-combustion boiler power generation system. U.S. Pat. Nos. 6,048,472; 6,110,979; 6,114,400; 6,296,686; 7,261,751; and 8,349,214 disclose different oxygen transport membrane syngas reactor configurations that convert methane into synthesis gas by reactions with oxygen supplied from low pressure air across the membrane, and in the presence of reforming catalysts. This technology avoids the need for additional air separation unit plant oxygen to react the methane, and requires a lower amount of fuel-fired heat to support the reforming reactions. The product synthesis gas from the oxygen transport membrane syngas reactor has typically greater than 90% of the methane in the feed reacted to form a synthesis gas comprising hydrogen and CO. This synthesis gas, with reduced methane, can then be utilized downstream in either a combined cycle utilizing gas and steam turbines or full oxy-combustion power generation cycle while facilitating carbon capture. In the case of the combined cycle configuration, a slip stream of the separated hydrogen post acid gas removal may be used for providing the heat to carry out endothermic reforming reactions in oxygen transport membrane syngas system thus avoiding additional CO2 emissions.